At this point everything is working and in mongo the set values get stored.

In this second step I would like to get the value from the database
into a different controller to include them in above mentioned
formula. The only thing working so far is to recall the values like
this in the html view directly:

So the idea behind the whole MEAN stack and more-or-less the whole frontend/backend idea is that your angular application is the frontend and you need to make a AJAX call to your Node.js service to connect to MongoDB

Think of it like this
Angular <---AJAX---> Node <------> Mongo

So the first thing you need is to build a rest service (using the help of the express framework) so something like this would work